To clarify the description, existing technologies and the technical problems they encounter are initially described in general terms. After that, mention is made of various documents that illustrate those technologies. In the above-mentioned technical field, the invention relates to so-called “on-board” pilot's associate systems, i.e. systems that are located at least in part on board manned aircraft, such as helicopters or rotary wing convertible aircraft.
The invention also relates to so-called “remote” assistance. Under such circumstances, it applies to rotary wing drones, i.e. to unmanned rotorcraft. Thus, assistance in accordance with the invention may be given to some one other than a pilot since there is no-one on board the aircraft. Under such circumstances, it is given to a human operator controlling said drone remotely. More specifically, the invention relates to pilot's associate systems that provide terrain avoidance warning, known by the acronym TAWS for “Terrain Avoidance Warning System”. Such TAWSs need to make it possible to indicate dangerous obstacles situated ahead on the predicted trajectory of the aircraft, in a danger zone at a given instant, when setting closer.
In other words, such a system serves to produce warnings automatically as a function of a map whenever an obstacle in a danger zone in front of the aircraft interferes with the trajectory predicted for that aircraft at a given instant. Given the known coordinates of the instantaneous position of the aircraft, and also its flight plan and a map of the terrain it is overflying, a warning is issued whenever an obstacle interferes with the predicted avoidance trajectory, and takes a risk of making avoidance impossible.
When to trigger a warning is conventionally determined as a function of an avoidance trajectory considered as being possible for the aircraft, its initially predicted trajectory, and its instantaneous speed. In practice, it has been found that terrain avoidance warning systems, or other systems considered as Ground Proximity Warning Systems (GPWS), of the kind designed for airplanes are not satisfactory for rotary wing aircraft.
For example, patent EP 0 750 238, which has lapsed for lack of novelty, describes such a system for avoiding ground collision. That system is said to be adaptive. Although that system appears to be dedicated in general to aircraft of any type, it is appropriate only for airplanes. In particular, the system is not designed for a rotary wing aircraft or a helicopter. In addition, that document does not describe a conic section curve, nor even a proper conic section curve such as a parabola, an ellipse, or a hyperbola. That document does mention logic for updating data that incorporates parameters specific to the aircraft, and also a notion of “maneuvering capability”.
However the teaching of document EP 0 750 238 does not enable the instantaneous maneuverability of a rotary wing aircraft to be taken into account. Such a calculation as a function of up-to-date data (e.g. possible vertical acceleration and/or instantaneous mass) as produced by avionics is not described by that document. In an approach that is distinct, that document provides for the input and terrain altitudes to come from active terrain sensors, an inertial navigation system, and a radar altimeter.
This is associated with specific features of the structure and the operation of such rotary wing aircraft, where the influence of such features has a greater effect on the actual potential for avoiding obstacles with rotorcraft than it does with aircraft. A rotorcraft can perform many more different types of flight, than can a fixed wing aircraft. Apart from take-off and landing, with rotorcraft, only point-to-point transport flights are comparable with the flight of airplanes, in particular civilian airplanes. Thus, a given helicopter may perform close observation flights, tactical missions, life-saving missions, interventions on accidents, etc. During such flights, the parameters that are taken into consideration and the warnings that are delivered by the terrain avoidance system designed for an airplane are inappropriate, and possibly even undesirable or even dangerous. The same applies during stages of take-off and landing, during which pilot's associate systems designed for airplanes are bound to be inappropriate.
Given this observation, recommendations specific to helicopters have recently been prepared by a major consultative authority in aviation matters, namely the Radio Technical Commission Aeronautics (RTCA) relating to terrain avoidance warning systems. Those recommendations that are specific to helicopters recommend systems that are known are as HTAWSs.
With conventional terrain warning technologies for airplanes, the anticipation distance to an obstacle that implies modifying trajectory is calculated almost exclusively as a function of the absolute value of the forward speed of the airplane. In outline, the greater the value of this speed, the longer the anticipation distance. In other words, the faster the flight, the further in front of the airplane the terrain warning system performs its surveillance. Thus, said anticipation distance is a value that is expressed in units of length (e.g. meters or kilometers). Since it is within this system that the warning system verifies whether or not there exists a terrain obstacle, this distance in front of the aircraft is also known as the danger zone.
Conventionally, the anticipation distance is usually evaluated by multiplying the instantaneous speed of the airplane by a time constant that is applicable to an entire family of airplanes. This anticipation distance involves a transfer time, i.e. the estimated reaction time of the pilot, which is the time that elapses between the warning being issued and the pilot beginning to follow an avoidance trajectory.
Nevertheless, no other parameter concerning the flight (e.g. tactical, transfer, life-saving, etc.) is taken into account, so it happens all too often in practice that warnings are triggered in untimely manner or too frequently. This hinders the pilot rather than helping. As a result, to mitigate this hindrance, it happens that the pilot switches off the operation of the pilot associate system completely. This is particularly frequent when a terrain warning system designed for an airplane is adapted to a rotorcraft.
With such systems, the calculated avoidance trajectory also takes the form of a succession between a rectilinear segment that corresponds to the transfer time, followed by a circular arc directed away from the obstacle. The trajectory is said to be in the shape of a “ski tip”. In other words, most present systems rely in practice on a rectilinear transfer time based on the current speed, followed by a circularly arcuate avoidance curve of radius that corresponds to a maximum safety margin, without actually taking account of the real intrinsic capacity of the aircraft nor of its instantaneous situation. Naturally, the “ski tip” avoidance trajectory is calculated so that the pilot can act on the airplane and avoid the obstacle in the danger zone.
As mentioned above, because of the way the calculation is performed, it happens frequently in tactical flight that warnings are triggered in the absence of any real danger, or that they are erroneous or even practically permanent. From the above, it will be understood that it would be appropriate to provide a terrain warning system for a rotary wing aircraft that generates warnings only when they are of genuine use to the pilot, and at the most opportune moment possible, i.e. neither too soon nor too late. The term “reliability” is used to designate this selective exclusion of superfluous warnings.
In addition, it would be desirable for a terrain warning system for a rotary wing aircraft to provide safety that is increased, in the sense that a warning that can be avoided without recourse to the best or even maximum instantaneous capacity of the aircraft in question (i.e. its maneuverability), is inhibited or pushed back to a later moment. This enables a flight trajectory to be maintained that is as close as possible to the terrain without increasing the risks specific to the obstacles on that terrain. Such increased safety would be most desirable, e.g. during tactical military flying.
Nevertheless, it can be understood that the requirements of safety and the requirements of flying constraints are in opposition, since in practice the need is to devise a terrain warning system for a rotary wing aircraft that generates warnings specifically at the opportune moment while nevertheless remaining reliable and safe in terms of capacity for avoiding the obstacle.